Medium Range Air Quality Outlook

Mid-Atlantic Region

Issued: Sunday, July 12, 2015

Valid: July 13-17, 2015 (Monday – Friday)

Summary:

A modified air mass, mostly sunny afternoon skies, and stagnant conditions allow an Appreciable chance for USG ozone to continue for Monday at locations along the northern branch of the I-95 Corridor (PHL/TTN to NYC).  For the rest of the region, scattered showers/thunderstorms and increasing clouds associated with an advancing warm front will limit ozone production.  The forecast models are in much closer consensus today that an upper level trough will impact the Mid-Atlantic on Tuesday and Wednesday, with widespread unsettled conditions keeping ozone in the mostly Good range.  Canadian high pressure will build into the region for Thursday and Friday behind a progressive cold front, with a chance for ozone to reach the low/mid Moderate range along I-95 on Friday.

Discussion:

The weather models are in much closer consensus today on the main synoptic features for the medium range period.  The 12Z NAM, 06Z GFS, and 00Z ECMWF were consulted for this analysis, with a focus on a blend of the EC and GFS, per the WPC.  On Monday, the broad upper level ridge will still be in place in the western US, centered roughly over the Rockies/Plains.  The forecast models continue to show a “bundle” of shortwaves cresting the western ridge and beginning to dig out a longwave trough over the Great Lakes on Monday.  The models also have a strong shortwave/closed low over Hudson Bay.  The EC and GFS are in much better alignment today regarding the longwave trough, agreeing that it amplifies as it moves eastward on Tuesday and Wednesday.  The frontal boundary currently over NC will begin to move northward through the region today as a warm front, stalling in the vicinity of the Mason-Dixon Line (MDL) on Tuesday.  The forecast models bring several shortwaves over the Mid-Atlantic on Tuesday and Wednesday, which will trigger the development of waves of low pressure along the stalled surface frontal boundary.  On Wednesday, the Hudson Bay shortwave will pull a back door cold front into the northern Mid-Atlantic.  Canadian high pressure building southward will allow the back door cold front to catch up to the MDL stalled front and push the merged boundary down to the Carolinas on Thursday.  The EC and GFS still have some disagreement on how fast the upper level trough will exit, with the EC allowing it to linger into Thursday, but a blend of guidance, per WPC, brings zonal flow back over the Mid-Atlantic late Thursday into Friday.

Ozone is rising quickly this morning along the I-95 Corridor in ILG and PHL, which suggests the air mass in place over the northern and central parts of the region has become modified.  On Monday, the slowly moving warm front and developing longwave trough will trigger scattered showers and thunderstorms along the western portions of the Mid-Atlantic.  Although clouds will be on the increase across the region, the northern part of the I-95 Corridor will be partly to mostly sunny through the afternoon, which will allow for another day of possibly upper Moderate to isolated USG ozone, mainly for PHL/TTN to NYC.  Light onshore surface winds will assist in transporting highway emissions to locations north and west of I-95.  The air quality models have picked up on these weather conditions, and today’s 06Z runs and show USG to upper Moderate ozone along eastern PA and northern NJ/NYC metro area.  Based on this morning’s readings, another day of high ozone certainly seems possible on Monday.  The main forecast question will be how quickly optically thick clouds move in from the west/southwest, ahead of the approaching warm front.  Back trajectories are stagnant and recirculating, so no significant changes to air mass characteristics are expected.  Particles remain in the lower end of the Moderate range today – likely in response to stagnation – despite a surge of drier air that built into the region yesterday.  Persistent Moderate particle concentrations seem probable on Monday.

Tuesday and Wednesday still look very unsettled, with mostly cloudy skies and scattered showers and thunderstorms.  Periods of rain may be locally heavy.  It will be warm and humid as tomorrow’s warm front stalls near the MDL and waves of low pressure form along it.  Ozone will return to the Good range across the region.   Particles may linger in the Moderate range in some areas on Tuesday, depending on how strong the southerly flow and precipitation are, but a gradual return to Good conditions is expected for particles as well.

The EC and GFS are coming into consensus that Thursday will be post-frontal, with slightly cooler and drier air filtering in on northerly flow.  Canadian surface high pressure will ease into the Mid-Atlantic from ON.  The sun will return, but a presumably clean air mass should limit ozone and particles to the Good range for another day.

With a return to zonal flow at 500 mb on Friday, the flow at 850 mb will shift west/southwesterly.  Temperatures will be around average.  Mostly sunny skies and light surface winds will likely allow ozone to rebound into the low/mid Moderate range at the usual locations along the I-95 Corridor.

-Huff